AVP is known to have problems with false-positive detections. It is entirely possible (and probable, if you downloaded your fresh copy of the Manager installer directly from manager.io over a secure connection onto a computer that you know to be malware free) that this is just a false-positive – a false alarm.
First and foremost, you (and @lubos) should file a false-positive report with AVG so they can fix the issue. Here’s the reporting form: False Positive File Form.
Please also read this AVG Forum page for some background: https://support.avg.com/answers?id=906b0000000D5jyAAC
All that said, you should be very circumspect about clicking Allow unless you are absolutely, positively certain that you have a pristine copy of the Manager installer. You can do that by asking @Lubos for the MD5/SHA checksum of the current installer and then comparing it to the checksum of the file you’ve downloaded by using an online checksum generator. The checksums should match exactly. Although we can be confident that the files on manager.io are clean (or at least as confident as any of us can be given that we trust @Lubos and his company), it is possible that if your computer is already infected, it could be redirecting your manager.io download requests to another, malicious site or inserting malicious code on the fly into anything you download. You can never be too careful.